feat(syscall): add x86_64 raw syscall layer, errno, internal ABI headers

src/internal/syscall_ret.c

@@ -0,0 +1,23 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include "errno.h"
+#include "syscall.h"
+
+/*
+ * Translate a raw syscall result into the C convention: return the value on
+ * success, or store the negated errno and return -1 on error. The -4095
+ * bound is the kernel's MAX_ERRNO: any result at or below -4096 is a
+ * legitimate value, not an error code.
+ */
+int
+syscall_ret(long r)
+{
+    if (r < 0 && r > -4096)
+    {
+        errno = (int)-r;
+        return -1;
+    }
+    return (int)r;
+}